Most people belong to one organization and never see this. If you belong to more than one — you manage compliance for two companies, or you're an agency user working inside a client — a switcher appears in the sidebar.
Switching
Use the organization control in the sidebar and pick another. The whole app follows: contractors, documents, settings and alerts are all scoped to whichever organization you're in.
The control only appears when you can reach more than one. If you don't see it, you belong to one organization.
Careful
Separate, not shared
Organizations don't share anything. Separate contractors, separate documents, separate requirements, separate billing. A contractor who works for both companies is two records, and that's correct — each company holds its own certificate, naming itself as certificate holder.
For agencies
If you work for an agency, you don't switch organizations — you open a client from the console, which is a different and more deliberate action. See Open a client and work inside it.